CriticalCVE-2023-28771

    Zyxel Multiple Firewalls OS Command Injection Vulnerability (CVE-2023-28771)

    Published In CISA KEV since Sources: SingCERT, pwn.watch, CVE.org

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    What is affected, what to do

    This vulnerability allows an unauthenticated attacker to execute OS commands on Zyxel firewalls, including ATP, USG FLEX, VPN, and ZyWALL/USG, by sending crafted packets. It affects firmware versions 4.60 to 5.36 and is actively exploited in the wild. The vulnerability is listed in the CISA KEV catalog, indicating the need for immediate action. Organizations should update their devices to the latest firmware version to mitigate the risk.

    The vulnerability is being actively exploited; treat it as a priority. The available sources name no fixed version. Check the vendor guidance in the references below. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Zyxel ZyWALL/USG series firmware eq 4.60 through 4.73..4.60 through 4.73
    • Zyxel VPN series firmware eq 4.60 through 5.35..4.60 through 5.35
    • Zyxel USG FLEX series firmware eq 4.60 through 5.35..4.60 through 5.35
    • Zyxel ATP series firmware eq 4.60 through 5.35..4.60 through 5.35

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    9.8

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    9.8

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: SingCERT, pwn.watch, CVE.org

    99.28 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.94

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    231.43 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    actively exploitedSource: CIRCL KEV How we classify
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    9.8 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    9.8
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    99.28 %
    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    99.94
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    CISA assessment (SSVC)
    Exploitation: active · automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 18 November 2023
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-78

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This bug is network-reachable without authentication and fires from a single crafted packet against the very appliance that exposes VPN services to the internet. Shortly after disclosure it was mass-scanned and exploited at scale, chiefly by botnet campaigns enlisting devices for DDoS, and it sits among the CVEs most likely to be exploited. Typical victims are SMEs, branch networks and remote sites that use Zyxel firewalls as their default VPN gateway. CISA does not flag known ransomware use, but a compromised perimeter device is a prime initial-access foothold. Treat every internet-facing, unpatched unit as a critical emergency that outranks almost any other open finding under NIS2. Internal-only devices with IKE disabled can follow the regular patch cycle.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory: Enumerate every Zyxel ATP, USG FLEX, VPN and ZyWALL/USG appliance and record its firmware version (dashboard or CLI show version). Vulnerable ranges are 4.60–5.35 for ATP/USG FLEX/VPN and 4.60–4.73 for ZyWALL/USG.
      • Patch: Upgrade ATP, USG FLEX and VPN units to ZLD 5.36 or later. Per the context, ATP100/200/500 are fixed in 5.36. For ZyWALL/USG, a patch is available from the vendor — look up the exact version in the Zyxel security advisory for CVE-2023-28771. Internet-facing units go first.
      • Shrink the attack surface if you cannot patch right now: The flaw sits in IKE packet handling, i.e. the VPN stack. Disable unused IPsec VPN gateways or block inbound UDP/500 and UDP/4500 on the WAN interface. Where site-to-site VPN is in use, restrict these ports to known peer IPs.
      • Compromise check before and after patching: Export the running config and diff it against the last known-good copy. Look for new admin accounts, altered routes or NAT rules, unknown scheduled tasks, and management services (SSH/HTTPS) enabled on WAN.
      • If compromise is suspected: Treat the device as untrusted. Reflash the firmware, restore the config from a clean backup, and rotate every credential held on the box (admin, VPN PSKs, certificates, LDAP/RADIUS bind accounts). Then hunt for lateral movement behind it.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation / perimeter: An upstream router ACL should permit UDP/500 and UDP/4500 only from defined VPN peers. For remote-access VPN with dynamic client IPs, temporarily move users to SSL VPN or an alternate concentrator.
      • Management plane: Turn off web GUI, SSH and SNMP on the WAN interface. Allow administration only from a dedicated management VLAN or jump host.
      • IPS / network sensors: Deploy a Suricata/Snort rule on UDP/500 that flags IKEv2 packets (major version 2) carrying Notify payloads with shell metacharacters (;, |, `, $() or strings such as wget, curl, /tmp/, sh -c. Enable the current ET Open/ET Pro signatures for CVE-2023-28771.
      • Egress control for the firewall itself: Restrict traffic originating from the appliance to required destinations only (Zyxel update servers, NTP, DNS, syslog). Block arbitrary outbound HTTP/HTTPS/TFTP/Telnet so attackers cannot pull payloads or reach botnet C2.
      • Logging: Forward the device's syslog to the SIEM, including the IKE/IPsec and system categories. Local logs cannot be trusted after compromise.

    3. Runbook step 3

      Detection rules

      • Network (Zeek/Suricata): Inbound IKE on UDP/500 from sources with no established peer relationship, especially single packets not followed by a complete IKE_SA_INIT exchange. Example in Zeek: conn.log where id.resp_p == 500 && proto == "udp" && id.orig_h !in known_vpn_peers.
      • Firewall-originated outbound traffic: The appliance's own IP initiating HTTP/TFTP/Telnet sessions to external hosts. Example in KQL: CommonSecurityLog | where SourceIP in (zyxel_wan_ips) and DestinationPort in (80,69,23,8080) and not(DestinationIP in (allowlist)).
      • Device syslog: Bursts of IKE malformed or invalid-packet errors from unknown peers, followed by unexpected system events such as reboots, process crashes or config changes. Sigma shape: logsource: product=zyxel, service=syslog | selection: msg|contains: ['IKE', 'malformed'] | timeframe: 5m | count() by src_ip > 3.
      • Configuration drift: Run a daily diff of the exported config against a baseline. Alert on new users, privilege changes, and new NAT or firewall rules pointing at internal hosts.
      • Availability anomalies: Unplanned reboots, CPU spikes or flapping VPN tunnels in SNMP/NMS monitoring can indicate exploitation attempts or botnet load.

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    Improper error message handling in Zyxel ZyWALL/USG series firmware versions 4.60 through 4.73, VPN series firmware versions 4.60 through 5.35, USG FLEX series firmware versions 4.60 through 5.35, and ATP series firmware versions 4.60 through 5.35, which could allow an unauthenticated attacker to execute some OS commands remotely by sending crafted packets to an affected device.

    Source: CSA/SingCERT, pwn.watch, NVD (NIST), cvelistv5, CISA KEV Catalog, CVE.org (MITRE)

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    ZyxelATP series firmware
    • 4.60 through 5.35
    • —
    ZyxelUSG FLEX series firmware
    • 4.60 through 5.35
    • —
    ZyxelVPN series firmware
    • 4.60 through 5.35
    • —
    ZyxelZyWALL/USG series firmware
    • 4.60 through 4.73
    • —

    Vendors and products

    Affected operating systems

    Affected operating systems
    Otherzyxelatp100_firmware—
    Otherzyxelatp100w_firmware—
    Otherzyxelatp200_firmware—
    Otherzyxelatp500_firmware—
    Otherzyxelatp700_firmware—
    Otherzyxelatp800_firmware—
    Otherzyxelusg_flex_100_firmware—
    Otherzyxelusg_flex_100w_firmware—
    Otherzyxelusg_flex_200_firmware—
    Otherzyxelusg_flex_500_firmware—
    Otherzyxelusg_flex_50_firmware—
    Otherzyxelusg_flex_50w_firmware—
    Otherzyxelusg_flex_700_firmware—
    Otherzyxelvpn1000_firmware—
    Otherzyxelvpn100_firmware—
    Otherzyxelvpn300_firmware—
    Otherzyxelvpn50_firmware—
    Otherzyxelzywall_usg_100_firmware4.73
    Otherzyxelzywall_usg_100_firmware—
    Otherzyxelzywall_usg_310_firmware4.73
    Otherzyxelzywall_usg_310_firmware—

    Weakness classes (CWE)

    • CWE-78Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')

      Base

      The product constructs all or part of an OS command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended OS command when it is sent to a downstream component.

      Entry at MITRE CWE-78

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited
    CIRCL KEVvulnerability.circl.lu (opens in a new tab)actively exploited
    Packet StormZyxel IKE Packet Decoder Unauthenticated Remote Code Execution (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—

    References & sources

    • zyxel.comVendor advisory: Zyxel Multiple FirewallsVendor advisory, pwn-watch
    • packetstormsecurity.comfiles/172820/Zyxel-IKE-Packet-Decoder-Unauthenticated-Remote-Co…Exploit, Third-party advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2023-28771government-resource, US government resource
    • pwn.watchpwn.watch: Zyxel Multiple Firewalls: OS Command Injection (CVE-2023-28771)pwn-watch
    • NVD (NIST)NVD: CVE-2023-28771pwn-watch
    • cisa.govCISA Known Exploited Vulnerabilities Catalogpwn-watch
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